The dead bug is a good exercise to train the control of pelvic position with isometric ab contraction.
Lie on your back and crunch the abs to bring the tailbone and shoulders up off the floor. This crunched position needs to be maintained throughout the set—the point is to fight against the pelvis rotating forward as the legs extend and place tension on it.
The dead bug is a good way to strengthen the ability of the abs to maintain a spine-pelvis relationship against forces trying to move them. It’s important that the spine is set in position and not allowed to move for the sake of straighter legs or greater range of motion or the purpose is defeated.
The dead bug can be done with straight legs to increase difficulty, or the range of motion shortened with bent knees to make it even easier. Small weights or bands can also be added.
The dead bug should generally be done for 2-4 sets of 5-10 or more reps per side. This is a good exercise for training preparation, or in addition to heavier, lower-rep or more difficult ab work in a given workout or in a circuit.